Photonic local oscillator signal generator and method for generating a local oscillator signal

ABSTRACT

The present invention provides a photonic local oscillator signal generator. The generator includes a plurality of waveguides fabricated in a single substrate. The waveguides are fabricated with a lateral separation that enables an evanescent tail of an optical mode field generated in one waveguide to overlap with an evanescent tail of an optical mode field generated in an adjacent waveguide. The overlap of the evanescent tails produce cross-coupling between the laser light generated in the adjacent waveguides. The generator uses a common pumping element to pump laser cavities of both of the waveguides. The use of a common pumping element and the cross-coupling produces phase locking between the laser light generated in the plurality of waveguides. As a result of the phase locking, mutual optical coherence between the laser light is achieved. The mutual optical coherence provides an interference signal upon photodetection. This interference signal is the offset between the center frequency of the laser light produced in the adjacent waveguides and serves as the local oscillator signal.

FIELD OF THE INVENTION

The present invention relates to local oscillator signal generators andgeneration of local oscillator signals. More particularly, the presentinvention relates to a photonic local oscillator generator and thegeneration of a local oscillator signal therefrom in the microwavefrequency band and higher.

BACKGROUND OF THE INVENTION

Traditional frequency synthesis electronics are not adequate forgeneration of microwave and higher frequency local oscillator signals.Presently, high frequency stable electrical signals from localoscillators are obtained by multiplying a low-frequency reference (e.g.,quartz oscillator) to the required high frequency, for example, 32 GHz,with several stages of multipliers and amplifiers. The resulting systemis bulky, complicated, inefficient, presents high phase noise, and iscostly.

Typical systems require the generation of multiple local oscillatorsignals followed by distribution of the signals to various pointsthroughout an electronics rack. The cabling required for thisdistribution presents high loss and distortion for the local oscillatorfrequencies in the microwave and higher frequency bands. This oftenrequires the duplication of costly frequency synthesis hardware atmultiple points in the system. Alternate local oscillator signalgenerating proposals require complex frequency-locking and phase-lockingelectronics systems to preserve signal integrity.

Photonic generation of local oscillator signals provides improvedspectral purity of the generated signal. Photonic generation is capableof producing local oscillator signals up to several Terahertz infrequency. While capable of producing these high frequency localoscillator signals, the known techniques for generating these signalsproduce unstable signals and require external electronic feedback loopsto stabilize the generated signals.

SUMMARY OF THE INVENTION

The present invention is a photonic local oscillator signal generatorwhich includes a plurality of parallel optical channel waveguidesfabricated in a single substrate such that an evanescent tail of anoptical mode field created by laser light generation in each of theplurality of waveguides overlaps with an evanescent tail of an opticalmode field created by laser light generation in an adjacent waveguide.The generator also includes a pan-chromatic mirror attached to an end ofthe substrate. The plurality of waveguides is fabricated within thesubstrate such that the pan-chromatic mirror delimits one end of a lasercavity of each of the plurality of waveguides. The generator alsoincludes a Bragg grating mirror which has a plurality of gratingfringes. The grating mirror delimits a second end of the laser cavity ofeach of the plurality of waveguides. The grating fringes are spacedapart from each other and traverse the plurality of waveguides such thatthe spacing between the grating fringes, as the grating fringes traverseeach of the plurality of waveguides, is different.

A local oscillator signal is generated by pumping the plurality ofoptical channel waveguides with a single pumping means, generating anoptical mode field in the laser cavity of each of the plurality ofwaveguides, each of the optical mode fields having a unique centerfrequency, phase locking the optical mode field of adjacent ones of theplurality of waveguides, and photodetecting an interference signalresultant from the phase locking of the adjacent optical mode fields.

The present invention provides a local oscillator signal generator forgenerating local oscillator signals ranging in frequency from a fewmegahertz to several terahertz.

The present invention also provides a local oscillator signal generatorwith improved resistance to environmental variations.

The present invention further provides a local oscillator signalgenerator that generates highly stable signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a photonic local oscillator signalgenerator of the present invention.

FIGS. 2(a)-2(d) are graphs representing laser light frequenciesproducible by the generator of FIG. 1.

FIG. 3 is a perspective view of another embodiment of a photonic localoscillator signal generator of the present invention.

DESCRIPTION OF THE INVENTION

In the drawings, where like numerals identify like elements, there isshown a local oscillator signal generator generally designated by thenumeral 10. The generator 10 illustrated in FIG. 1 includes a substrate12 and at least two optical channel waveguides 14, 16 fabricated in thesubstrate 12. The waveguides 14,16 are typically fabricated by dopingthe substrate 12 with rare earth ions such as erbium, ytterbium, orneodymium. The substrate 12 may also be any optical waveguide materialincluding semiconductor, lithium niobate, glass, or optical polymer. Thewaveguides 14, 16 are fabricated parallel to each other in the substrate12.

The generator 10 also includes a pan-chromatic mirror 18. Thepan-chromatic mirror 18 is attached to one side of the substrate 12 andforms or delimits a first end 19 a, 19 b of a laser cavity 20 a, 20 b ofeach of the waveguides 14, 16. The pan-chromatic mirror 18 effectivelyreflects all wavelengths of light equally. A second end 21 a, 21 b ofeach of the laser cavities 20 a, 20 b, opposite to the first end 19 a,19 b of the laser cavity 20 a, 20 b, is delimited by a Bragg gratingmirror 22. The laser cavity 20 a, 20 b for each of the waveguides 14, 16therefore has a length L that is equal to the distance between thepan-chromatic mirror 18 and the Bragg grating mirror 22. The length L ofeach of the laser cavities 20 a, 20 b is one of the parameters thatdetermines the wavelength/frequency of light generated in a particularlaser cavity, as discussed in more detail below.

The Bragg grating mirror 22 includes a plurality of grating fringes 24.The grating fringes 24 traverse each of the waveguides 14, 16. Thegrating fringes 24 are spaced apart from each other by an amount P.Furthermore, the grating fringes 24 are arranged in a fannedconfiguration. That is, as the grating fringes 24 traverse thewaveguides 14, 16 the spacing P between the grating fringes 24 willchange and will be different for each of the waveguides 14, 16. Thisresults in a different center frequency or wavelength for the laserlight generated in each of the waveguides 14, 16. The difference betweenthe center frequency/wavelength is based upon the rate of the gratingfan-out and the lateral separation between the waveguides 14, 16. Thisis based on simple trigonometry. As the rate of grating fan-out isincreased, the greater the separation between center frequencies foradjacent waveguides. Additionally, as the lateral separation betweenadjacent waveguides increases so will the difference between centerfrequencies. The center frequency of the waveguides may be tuned bymodifying the refractive index of the grating area of the waveguides 14,16. The refractive index may be changed by using any one of well-knowntechniques including thermal-optic, electro-optic, acoustic-optic oroptical-optic effects.

The output frequency/wavelength of an optical waveguide is defined by acombination of the laser transition line of the gain medium, i.e.,material within the laser cavity, laser cavity resonance condition, andlaser cavity mirror reflectivity. FIG. 2 illustrates the role each ofthese individual factors plays and the resultant laser spectra thatresult from the combined effect of these factors. The laser transitionline spectrum is defined by the dopant and substrate materials. Thisspectrum, as shown in FIG. 2(a), defines the complete range of opticalfrequencies the laser waveguide is capable of generating. The lasercavity 20 is that part of a waveguide that is capable of resonantlyaccumulating optical gain, i.e., located between the pan-chromaticmirror 18 and the grating mirror 22. The length L of the laser cavity 20determines the allowable optical field modes. The laser cavity canproduce laser light of multiple frequencies separated by c/2L. Thesefrequencies define the available modes that can be produced in the lasercavities. In this formulation “c” is the speed of light. These discretemodes are illustrated in FIG. 2(b). The combination of the transitionline and cavity spectra reveals that there is a finite range of discretelaser frequencies that the laser can generate.

Selection of single frequencies from among the several that can begenerated from the cavity 20 is achieved by using a mirror or mirrorsthat reflect light only in very narrow frequency bands. FIG. 2(c)illustrates a graph showing the pan-chromatic mirror 18 as reflectingall optical frequencies substantially identically. FIG. 2(c) alsoillustrates the frequency bands for the Bragg grating mirror 22 for thewaveguides 14, 16. These bands are the results of the preselectedintersection of the Bragg mirror 22 and the waveguides 14, 16. The Braggmirror 22 is highly frequency selective and allows only a single opticalmode to exist in each laser cavity 20. By fanning the grating fringes24, it is assured that the frequency for the laser light generated ineach laser cavity is different without limiting the offset, S, betweenthe frequencies of laser light generated in each laser cavity 20. Asdiscussed above, the center frequency of the grating mirror 22 for eachof the waveguides 14, 16 may be adjusted or tuned by modifying therefractive index of the grating area. The offset, S, between thefrequency of laser light generated in adjacent laser cavities 20 will bethe local oscillator signal generated upon photodetection of thecombined waveguide signals. As discussed below, the local oscillatorsignal is equal to the heterodyne beat frequency or beat signalresulting from the interference of the laser light generated in adjacentlaser cavities 20. FIG. 2(d) illustrates the composite frequencyspectrum of the output signal of the waveguides of the local oscillatorgenerator 10. As illustrated in FIG. 2(d), laser light having a uniquecenter frequency is generated in each laser cavity 20 a, 20 b.

The waveguides 14, 16 must be located within the substrate 12 to enablethe evanescent tail of the optical mode field generated in each of thelaser cavities to overlap with the evanescent tail of the optical modefield generated in an adjacent laser cavity. The size of the opticalmode field and the concomitant evanescent tail is a function of thefrequency/wavelength of the laser light generated in the laser cavity20. Therefore, the lateral separation between the waveguides 14, 16 is afunction of the frequency/wavelength of the laser light generated in thelaser cavities 20. As a result, the desired center frequency or range ofcenter frequencies for tunable devices for each waveguide 14, 16 must bepreselected prior to fabrication. Once the center frequency for eachwaveguide 14, 16 has been preselected. the lateral spacing betweenadjacent waveguides can be determined based upon the optical mode fieldgenerated by the preselected center frequency.

The generator 10 includes an optical combiner/splitter 26 that couplesthe waveguides 14, 16. The generator 10 may also include an opticalfiber 28. The optical fiber 28 couples light from a pumping element (notshown) to the optical combiner/splitter 26 and into the waveguides 14,16. The pump light excites the gain medium i.e., the material of each ofthe laser cavities 20 a, 20 b, to generate laser light. Pumping both thewaveguides 14, 16 with the same pump element promotes phase locking ofthe laser light generated in the waveguides 14, 16. This process iscommonly known as injection-locking. As the pumping element suppliespumping light to the laser cavities 20 a, 20 b, they generate laserlight. An optical mode field is created for each laser cavity 20 uponthe generation of the laser light. Each optical mode field includes anevanescent tail. The optical mode field created within one of thewaveguides 14 includes an evanescent tail 30 that occupies the spacedelimited by circle 32 along the laser cavity 20 a. The optical modefield created within another one of the waveguides 16 includes anevanescent tail 34 that occupies the space delimited by circle 36 alongthe laser cavity 20 b. The evanescent tail 30 of one waveguide 14overlaps with the evanescent tail 34 of another waveguide 16 in the area37 defined by the overlap of circle 32 and circle 36 along the lasercavities 20 a, 20 b. This overlap enables cross-coupling between thelaser cavities 20 a, 20 b. The cross-coupling between the laser cavities20 a, 20 b produces mutual optical coherence between the laser lightgenerated in one waveguide 14 and the laser light generated in the otherwaveguide 16.

The size of the evanescent tail generated by each of the waveguides 14,16 is dependent upon a variety of factors, including the substratematerial, the center frequency selected and the pumping light. Thesefactors are well known to those skilled in the art. Once the waveguideparameters have been selected and determined, the size of the evanescenttail for each of the waveguides can be determined. Once the size of theevanescent tail is determined, the spacing between the waveguides in thesubstrate can be determined. As discussed above, the waveguides shouldbe spaced to allow overlap of adjacent evanescent tails.

Each waveguide produces self-coherent laser light. Typically, laserlight generated in two different waveguides will not be mutuallycoherent. In order for mutual optical coherence to occur between laserlight generated in different waveguides, additional circumstances needto be present. These may include evanescent tail cross-coupling of thewaveguides and pumping of both laser cavities with a common pumpingelement. When mutual optical coherence is obtained, it causesinterference between the laser light of different wavelengths generatedin the waveguides 14, 16 upon photodetection. In contrast to twoincoherent laser beams, which generate two independent signals whenphotodetected (because there is no interference between them), twocoherent laser beams produce three signals when photodetected: twosignals that are independent and correspond to the incoherent case, anda third signal at a frequency equal to the offset, S, between the centerfrequencies of the two, mutually coherent beams. The third signal iscommonly referred to as the heterodyne beat frequency or beat signal.

Once the laser cavities 20 a, 20 b generate the laser light, the laserlight is output through the optical combiner/splitter 26 to the opticalfiber 28 and fed to a photodetector 38.

Locating the waveguides 14, 16 in a single substrate reduces the effectof environmental variations. For example, if the refractive index of theentire substrate changes, then any center frequency drift that resultsin one waveguide will also occur in the other waveguide. This, in turn,preserves the offset frequency between the two waveguides.

By fabricating additional waveguides in the substrate additional localoscillator signals may be generated from a single generator. Due towaveguide properties, if all of the adjacent waveguides have overlappingevanescent tails, once light generated in two of the waveguides ismutually optically coherent than cross coupling will be present betweenall adjacent waveguides and light generated in all of the waveguideswill be mutually optically coherent. This commutation extends to anynumber of waveguides so long as each adjacent pair is mutually opticallycoherent.

An alternate embodiment, illustrated in FIG. 3, implements an example ofsuch a generator. The generator 10′ includes an array of waveguides 39.In this embodiment the generator 10′ has four waveguides 40, 42, 44, and46. The number of waveguides fabricated in the substrate is limited onlyby available technology and economic concerns. Each of the waveguides40, 42, 44, and 46 is fabricated and positioned in accordance with thedescription of the embodiment illustrated in FIG. 1. The waveguides 40,42, 44, and 46 are mutually optically coherent due to the evanescenttail coupling between the adjacent waveguides as illustrated by theoverlap 48, 50, and 52 of the evanescent tails 54, 56, 58, and 60 of thewaveguides 40, 42, 44, and 46, respectively, as delimited by circles 62,64, 66, and 68, respectively. The generator 10′ also includes twooptical combiner/splitters 70, 72 that couple pumping light into each ofthe waveguides 40, 42, 44, and 46 and couple generated laser light tothe optical fibers 76, 78 or a pair of photodetectors (not shown). Thegenerator 10′ also includes a cross-connect switch 80 (also known as anoptical space switch) that selectively couples generated laser lightfrom any one of the four waveguides, for example 42, into one leg of oneof the optical combiners, for example 70, and couples generated laserlight from another one of the four waveguides, for example 46, into theother leg of the same optical combiner, for example, 70. This enablesthe generator 10′ to generate multiple local oscillator signals (onefrom each combiner/splitter) having a variety of center frequencies byselecting two waveguides, for example 40 and 42 or 42 and 46, whereineach generated local oscillator signal will have a frequency equal tothe offset between the center frequencies of the selected waveguides.The generator may have as many combiner/splitters as is necessary andpractical to produce as many local oscillator frequencies as desired.

The switch 80 and the optical combiner/splitters 70, 72 may beintegrated with the generator 10′ or may be discrete elements.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

What is claimed is:
 1. A photonic local oscillator signal generatorcomprising: a substrate; a plurality of optical channel waveguidesfabricated in the substrate; a pan-chromatic mirror attached to thesubstrate delimiting a first end of a laser cavity of each of theplurality of waveguides; and a Bragg grating mirror delimiting a secondend of the laser cavity of each of the plurality of waveguides, thegrating mirror including a plurality of grating fringes, the spacingbetween the grating fringes being different for each of the plurality ofwaveguides; wherein the plurality of waveguides are fabricated such thatin operation of the signal generator an evanescent tail of an opticalmode field created by laser light generation in each of the plurality oflaser cavities overlaps with an evanescent tail of an optical mode fieldcreated by laser light generation in an adjacent laser cavity.
 2. Aphotonic local oscillator signal generator as recited in claim 1,wherein the spacing between each of the plurality of grating fringesvaries linearly as the grating fringes traverse the plurality ofwaveguides.
 3. A photonic local oscillator signal generator as recitedin claim 1, wherein the plurality of waveguides are fabricated parallelto each other.
 4. A photonic local oscillator signal generator asrecited in claim 1, further comprising a cross-connect switch connectedto each of the plurality of waveguides.
 5. A photonic local oscillatorsignal generator as recited in claim 1, further comprising a singlepumping means for pumping the plurality of waveguides.
 6. A photoniclocal oscillator signal generator comprising: a substrate; a pluralityof optical channel waveguides fabricated in the substrate, the pluralityof optical channel waveguides fabricated parallel to each other suchthat an evanescent tail of an optical mode field created by laser lightgeneration in each of the plurality of waveguides overlaps with anevanescent tail of an optical mode field created by laser lightgeneration in an adjacent waveguide; a pan-chromatic mirror attached tothe substrate delimiting a first end of a laser cavity of each of theplurality of optical channel waveguides; and a Bragg grating mirrordelimiting a second end of the laser cavity of each of the plurality ofoptical channel waveguides, wherein the Bragg grating mirror includes aplurality of grating fringes, the grating fringes being distinctlyspaced from each other as they traverse the plurality of waveguides. 7.A photonic local oscillator signal generator as recited in claim 6,wherein the spacing between each of the plurality of grating fringesvaries linearly as the grating fringes traverse the plurality ofwaveguides.
 8. A photonic local oscillator signal generator as recitedin claim 6, further comprising a cross-connect switch connected to eachof the plurality of waveguides.
 9. A photonic local oscillator signalgenerator as recited in claim 6, further comprising a single pumpingmeans for pumping the plurality of optical channel waveguides.
 10. Aphotonic local oscillator signal generator comprising: a substrate; aplurality of optical channel waveguides fabricated in the substrate; apan-chromatic mirror attached to the substrate delimiting a first end ofa laser cavity of each of the plurality of optical channel waveguides; aBragg grating mirror delimiting a second end of the laser cavity of eachof the plurality of optical channel waveguides; and a single pumpingmeans for pumping each of the plurality of optical channel waveguides;each of the plurality of laser cavities generating laser light having aunique center frequency when pumped; and an optical mode field beingassociated with the laser light in each of the plurality of lasercavities, each optical mode field having an evanescent tail whichoverlaps with the evanescent tail of the adjacent optical mode field.11. A photonic local oscillator signal generator as recited in claim 10,wherein the grating mirror includes a plurality of grating fringes, thegrating fringes spaced from each other and traversing the plurality ofwaveguides such that the spacing between the grating fringes for each ofthe plurality of waveguides is different.
 12. A photonic localoscillator signal generator as recited in claim 11, wherein spacingbetween each of the plurality of grating fringes varies linearly as thegrating fringes traverse the plurality of waveguides.
 13. A photoniclocal oscillator signal generator as recited in claim 11, wherein thesignal generated by the overlap of adjacent evanescent tails is equal toa difference between the center frequency of the adjacent lasercavities.
 14. A photonic local oscillator signal generator as recited inclaim 10, wherein the plurality of optical channel waveguides arefabricated parallel to each other.
 15. A photonic local oscillatorsignal generator as recited in claim 10, further comprising aphotodetector for detecting a signal generated by the overlap ofadjacent evanescent tails.
 16. A photonic local oscillator signalproduced by the steps of: generating laser light in a first opticalchannel waveguide and a concomitant first mode field, the first opticalchannel waveguide laser light having a first center frequency, and thefirst mode field including an evanescent tail; generating laser light ina second optical channel waveguide and a concomitant second mode field,the second optical channel waveguide laser light having a second centerfrequency different from said first center frequency, and the secondmode field including an evanescent tail, the second optical channelwaveguide positioned relative to the first optical channel waveguidesuch that the first mode field evanescent tail and the second mode fieldevanescent tail overlap; and generating from the laser light a signalhaving a frequency equal to the difference between the first centerfrequency and the second center frequency.
 17. A photonic localoscillator signal as recited in claim 16, wherein overlapping the firstmode field evanescent tail and the second mode field evanescent tailproduces cross-coupling between the first and second optical channelwaveguides.
 18. A photonic local oscillator signal as recited in claim17, wherein the cross-coupling between the first and second opticalchannel waveguides produces optical coherence between the laser lightgenerated in the first optical channel waveguide and the laser lightgenerated in the second optical channel waveguide.
 19. A photonic localoscillator signal as recited in claim 16, further comprising the step ofdetecting a signal equal to the difference between the first centerfrequency and the second center frequency.
 20. A method of making aphotonic local oscillator signal generator, comprising the steps of:fabricating a plurality of parallel optical channel waveguides in asubstrate; attaching a mirror to the substrate thereby delimiting afirst end of a laser cavity within each of the plurality of waveguides;incorporating a Bragg grating mirror in the substrate thereby delimitinga second end of the laser cavity of each of the plurality of waveguides,the grating mirror traversing each of the plurality of waveguides andincluding a plurality of grating fringes; spacing the grating fringesfrom each other such that the spacing between the grating fringes acrosseach of the plurality of the waveguides is different; and positioningthe plurality of waveguides relative to each other such that anevanescent tail of an optical mode field created by laser lightgeneration in each of the plurality of laser cavities overlaps anevanescent tail of an optical mode field created by laser lightgeneration in an adjacent laser cavity.
 21. A method of making aphotonic local oscillator signal generator, comprising the steps of:fabricating a plurality of parallel optical channel waveguides in asubstrate; attaching a mirror to the substrate thereby delimiting afirst end of a laser cavity within each of the plurality of waveguides;incorporating a Bragg grating mirror in the substrate thereby delimitinga second end of the laser cavity of each of the plurality of waveguides,each of the plurality of laser cavities generating laser light having aunique center frequency when pumped, the grating mirror traversing eachof the plurality of waveguides and including a plurality of gratingfringes; and positioning the plurality of waveguides relative to eachother such that an evanescent tail of an optical mode field created bylaser light generation in each of the plurality of laser cavitiesoverlaps an evanescent tail of an optical mode field created by laserlight generation in an adjacent laser cavity.
 22. A method of making aphotonic local oscillator signal generator as recited in claim 21,further comprising the step of spacing the grating fringes from eachother such that the spacing between the grating fringes across each ofthe plurality of the waveguides is different.
 23. A method of generatinga photonic local oscillator signal, comprising the steps of: pumping aplurality of optical channel waveguides with a single pumping means;generating an optical mode field in a laser cavity of each of theplurality of waveguides, each optical mode field having a unique centerfrequency; phase locking the optical mode field of adjacent ones of theplurality of waveguides; photodetecting an interference signal resultantfrom the phase locking of the adjacent optical mode fields.